Sayyiduna Umar (may Allah be pleased with him) said: O people, be suspicious of (your own) opinion in matters of religion, for I saw myself rejecting the decision of the Messenger of Allah (peace and blessings be upon him) by my own opinion and reasoning (ijtihad), and by Allah, I was not falling short in seeking the truth. That was on the day of Abu Jandal, when the treaty was being written between the Messenger of Allah (peace and blessings be upon him) and the people of Makkah. He (peace and blessings be upon him) said: "Write: Bismillah al-Rahman al-Rahim (In the name of Allah, the Most Gracious, the Most Merciful)." They said: "Have we accepted what you say as true? Rather, write: Bismika Allahumma (In Your name, O Allah)." The Messenger of Allah (peace and blessings be upon him) agreed, but I refused, until the Messenger of Allah (peace and blessings be upon him) said to me: "You see that I agree, and you refuse?" Umar (may Allah be pleased with him) said: So I agreed.
Hadith Referenceالمعجم الكبير للطبراني / العشرة / 81